In a gas turbine engine, air is pressurized in a compression module during operation. The air channeled through the compression module is mixed with fuel in a combustor and ignited, generating hot combustion gases which flow through turbine stages that extract energy therefrom for powering the fan and compressor rotors and generating engine thrust to propel an aircraft in flight or to power a load, such as an electrical generator.
The compression system includes a rotor assembly comprising a plurality of rotor blades extending radially outward from a disk. More specifically, each rotor blade has a dovetail which engages with the disk, a platform forming a part of the flow path, and an airfoil extending radially from the platform to a tip. The platform may be made integral to the blade or, alternatively, made separately.
In some designs, the rotor blade, especially those in a fan rotor and the front stages of a multistage compression system, have a pair of circumferentially extending shrouds on the airfoil, one projecting from the pressure surface and one projecting from the suction surface. The shrouds are located at a radial location between the blade dovetail and the blade tip. In some other designs, the shrouds may be located at the tip of the blade airfoil. During normal operation of the compression system, the blades twist and the shrouds on adjacent blades contact with each other, forming a shroud ring that provides support to the blades. During engine operation, the shroud ring resists vibration and twisting of the blades. The term “midspan shroud” is used herein to refer to all supports on fan and compression system blades that contact with each other during operation, and includes all supports located anywhere on the span of the blade, including supports at the tip of the blade. The “midspan shrouds” as used herein, may be located anywhere along the blade span, not just at the midpoint of the span.
During certain abnormal events, such as a bird impact, other foreign object impact, or stalls during engine operation, the normal contact between the shrouds of adjacent blades is disturbed. The contact forces become high and misaligned due to the impacts and the shrouds become disengaged fully or partially. This is called “shingling” of the blades. Shingling causes significant wear and tear damage on the midspan shrouds. When the speed of the compressor rotor drops, the shingled blades may rebound, causing further wear and tear on the shrouds.
Fan or compressor blades sometimes have wear pads brazed on the contact faces of the midspan shrouds. Wear pads have been used on blades to address the wear problem. For example, some compressor blades contain a brazed-on WC—Co wear pad to reduce wear between two rubbing midspan shrouds.
The blades may comprise titanium or alloys thereof (i.e., Ti6Al-4V and/or Ti8Al-1V-1Mo alloys) having beta transus temperatures at or slightly above 1800° F. (about 982° C.). The wear pads are conventionally brazed to the titanium blade using a titanium-copper-nickel (TiCuNi) alloy braze foils. Diffusion occurs between TiCuNi braze foil and WC—Co wear pad during high temperature braze. Titanium forms brittle compounds with the alloying elements of the wear pad in the braze joint. As a result, the braze joint provides a high hardness (about 1200 KHN) W—Co—Ti—Cu—Ni alloy. The braze interface exhibits cracking at impact energies as low as 0.30 Joules, and the wear pad may be liberated from the substrate at the brittle braze interface at an impact energy of about 0.60 Joules.
Industrially available braze alloys have been unable to meet the demands for high ductility and low cost necessary for aircraft engine applications. Accordingly, there is a need for lower cost, high ductility, impact resistant brazing alloys for brazing WC—Co substrates to titanium or titanium alloy substrates. In particular, there is a need for brazing alloys for brazing WC—Co materials to titanium and titanium alloys without forming a brittle braze interface.